Hello Giacomo,
The SAR processing -> radiometric -> calibrate process converts the pixel data to actual backscattering values. It should be performed first in whatever processing chain you do. If you need to do polarimetric work after with your data, you need to check the box marked «Save in complex» when calibrating. After this step, your image has backscattering intensity pixels and is in ground range or slant range geometry.
The SAR processing -> Geometry -> Terrain correction -> Range Doppler Terrain Correction process corrects the geometric distorsions in the image caused by the SAR system (slant view, etc) and yields a geocoded image. After this step, your image has backscattering intensity pixels and is in a map coordinate system. This step is usually done at the end of the processing, depending on your application.
Any multilooking or speckle filtering is usually done between these two steps.
All this is taken from the SAR basics tutorial .
I hope this helps you.